High temperature diffusion furnaces are well known to the semiconductor industry. Heat treatment in high temperature diffusion furnaces is one of the many steps in the manufacturing process of silicon wafers. Typically, process gas is injected into the heat treatment process in order to alter the composition of the wafers. For example, heat treatment of wafers with an appropriate process gas allows doping elements such as boron to be introduced into the molecular structure of the semiconductor material.
A high temperature diffusion furnace may include multiple furnace modules capable of heat treating multiple sets of silicon wafers. The individual diffusion furnace modules comprise a heating element which includes a process chamber for a process tube which is shielded by a liner. The process chamber has a load end where the wafers are inserted and a source end where process gas is injected. In addition, scavenger boxes are included to remove excess process gas along with an energy kit. Typically, the furnace modules are housed in a stacked orientation.
The heating elements are generally cylindrical in shape and symmetrical. The heating elements have an outer metallic housing, usually comprised of stainless steel or aluminum, and inner layers of insulating materials such as ceramic fiber. A furnace chamber is created between heating coils and a heating element liner. Several heating coils are secured together to form a continuous coil, with the middle heating coil enabling optimal temperature at the middle of process tube. The end heating coil is operated to enable a temperature in the process tube sufficient to overcome losses out the end of the furnace and to preheat any gasses being introduced into the furnace. The heating coil is generally a helical resistance wire made of chrome-aluminum-iron alloy, the wires generally heavy gauge (0.289" to 0.375" in diameter) for extended heating coil life at an elevated temperature.
The maximum permissible operating temperature for the heating coil alloy is 1400.degree. C. Since a temperature differential exists between the heating coil and the inside of the process tube, diffusion furnaces are normally operated at a maximum operating process tube temperature of 1300.degree. C.
Silicon wafers are heat treated in the middle section of the process tube. The process tube is fabricated of quartz, polysilicon, silicon carbide or ceramic. The process tube in sheathed by a liner which separates the heating coils from the process tube creating a furnace chamber. The silicon wafers to be heat treated are mounted onto boats and loaded either manually or automatically into the center of the process tube from the load end of furnace. The boats used to hold the wafers are generally fabricated of quartz, polysilicon, silicon carbide or ceramic.
Multiple furnace module configurations have increased heat treatment process capability and flexibility. Separate heat treatment processes at various stages may be performed in separate process tubes simultaneously. However, a system incorporating a multiple furnace module configuration increases the likelihood that a process in one furnace module may be affected by processes in other furnace modules, Moreover, a failure of a single furnace module may effect the ability to process wafers in other furnace modules. These limitations of multiple furnace module designs, as well as heating element design limitations which affect the performance of the heat treatment process, are as follows: